Method of making discontinuous carbon fiber reinforced glass matrix composites with secondary matrix reinforcement

ABSTRACT

A discontinuous carbon fiber reinforced glass matrix composite includes a glass matrix, a plurality of carbon reinforcing fibers dispersed in the matrix, and a plurality of boron nitride reinforcing particles dispersed in the matrix. The composite may be fabricated by mixing glass powder and boron nitride reinforcing particles in a carrier liquid to create a slurry and adding a binder to the slurry. A continuous multifilament carbon fiber yarn is impregnated with the slurry and dried to remove the carrier liquid. The impregnated carbon fiber yarn is cut to a suitable length and is molded in a suitable molding means to form a carbon fiber reinforced glass matrix composite article.

This is a division of copending application Ser. No. 07/662,649 Nov. 01,1991, now U.S. Pat. No. 5,118,560.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to commonly assigned U.S. application Ser.No. 07/662,652 filed on Mar. 1, 1991 entitled, "Continuous Carbon FiberReinforced Glass Matrix Composites With Secondary Matrix Reinforcement."

TECHNICAL FIELD

This invention relates generally to carbon fiber reinforced glass matrixcomposites and particularly to glass matrix composites reinforced withdiscontinuous carbon fibers.

BACKGROUND ART

The use of carbon fiber reinforced glass matrix composites (CFRGMcomposites) as replacements for metal has become common in servicesrequiring high strength, stiffness, and light weight. Products in whichsuch composites have been used range from sporting goods to jet engines.

CFRGM composites typically comprise a glass or glass-ceramic matrix inwhich carbon fibers are imbedded. The carbon reinforcing fibers may beeither continuous or discontinuous depending on the desired application.Continuous fibers generally extend for the entire length of a compositearticle, while discontinuous fibers, which are significantly shorterthan continuous fibers, tend to provide more localized matrixreinforcement. As a result, continuous fiber CFRGM composites are oftenused for load bearing structural applications, while discontinuous fiberCFRGM composites are more suitable for nonload or low load bearingnonstructural applications, especially those in which parts must befabricated into complex shapes. Such composites are described incommonly assigned U.S. Pat. Nos. 4,314,852 to Brennan et al. and4,324,843 to Brennan et al. Articles made from carbon fiber reinforcedcomposites may be formed in several ways, including by hot pressing in ashaped die as taught in commonly assigned U.S. Pat. No. 4,314,852 toBrennan et al.; by transfer molding as taught in commonly assigned U.S.Pat. No. 4,428,763 to Layden; or by injection molding as taught incommonly assigned U.S. Pat. Nos. 4,464,192 to Layden et al. and4,780,432 to Minford et al.

The interaction between the carbon fibers and matrix material isresponsible for the superior properties displayed by CFRGM composites.The fibers contribute to the composite's strength and elastic modulus byabsorbing loads transferred from the matrix through fiber-matrixinterfacial bonds. The fibers improve the composite's toughness byinhibiting or blunting the formation of cracks in the matrix. Inaddition, carbon fibers exposed at the surface of the matrix imparttheir good lubricating properties to the composite.

Despite their superior physical properties, all CFRGM composites aresusceptible to carbon fiber oxidation, particularly when exposed toelevated temperatures. The problem is exacerbated by the presence ofmatrix microcracks which form during fabrication as a result of athermal expansion mismatch between the glass matrix and carbon fibers.Microcracking is especially extensive in discontinuously reinforcedcomposites because of the complex stress states arising from the randomthree-dimensional arrangement of the fibers. Matrix microcracks providechannels which permit oxygen to penetrate into the matrix, providing theopportunity for carbon fibers in the interior of the matrix to oxidizewhen exposed to elevated temperatures. Carbon fiber oxidation canquickly destroy the composite's strength and lubricity, making carbonfiber composites unsuitable for certain applications or requiringfrequent replacement of parts constructed from these composites.

Accordingly it would be desirable to have a discontinuously reinforcedCFRGM composite which resists the effects of oxidation and maintains itsstrength and lubricity, particularly at high temperatures.

DISCLOSURE OF THE INVENTION

The present invention is directed towards a discontinuously reinforcedCFRGM composite which resists the effects of oxidation and maintains itsstrength and lubricity, particularly at high temperatures.

One aspect of the invention includes a discontinuous carbon fiberreinforced glass matrix composite with a glass matrix, a plurality ofcarbon reinforcing fibers dispersed in the matrix, and a plurality ofboron nitride reinforcing particles dispersed in the matrix.

Another aspect of the invention includes a method of fabricating adiscontinuous carbon fiber reinforced glass matrix composite. Glasspowder and boron nitride reinforcing particles are mixed in a carrierliquid to create a slurry and a binder is added to the slurry. Acontinuous multifilament carbon fiber yarn is impregnated with theslurry and dried to remove the carrier liquid. The impregnated carbonfiber yarn is cut to a suitable length and is molded in a suitablemolding means to form a discontinuous carbon fiber reinforced glassmatrix composite.

Another aspect of the invention includes an article fabricated from theabove recited carbon fiber reinforced glass matrix composite.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying drawing.

BRIEF DESCRIPTION OF DRAWING

The Figure is a scanning electron micrograph which shows thedistribution of boron nitride particles in the matrix of a discontinuousCFRGM composite of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The matrix of the present invention may be any glass or glass-ceramicwhich imparts formability, thermal stability, and abrasion resistance tothe composite. Borosilicate glass, high silica content glass,aluminosilicate glass, and mixtures thereof possess these properties.Suitable glass-ceramic materials include lithium aluminosilicate andother conventional glass-ceramics such as aluminosilicate,barium-magnesium aluminosilicate, and combinations thereof.

Borosilicate glass is the preferred matrix material because it is moreeasily processed than other glass matrix materials and possessesreasonably good thermal stability. Suitable borosilicate glasses, suchas Corning Code 7070 and Corning Code 7740, are available from CorningGlass Works (Corning, N.Y.). Corning Code 7070 glass is especiallypreferred because its lower viscosity characteristics make fabricationof composite articles easier. Corning Code 7070 glass has a tensilemodulus of 7.4×10⁶ pounds per square inch (psi), a density of 2.13 gramsper cubic centimeter (g/cm³), a coefficient of thermal expansion (CTE)of 32×10⁻⁷ centimeter per centimeter per degree Celsius (cm/cm °C.), ananneal point of 496° C., a softening point of 760° C., and a workingpoint of 1068° C. Corning Code 7740 glass has a tensile modulus of9.1×10⁶ psi, a density of 2.23 g/cm³, a CTE of 32.5×10⁻⁷ cm/cm °C., andanneal point of 560° C., a softening point of 821° C., and a workingpoint of 1252° C.

The carbon reinforcing fibers may be any carbon fibers, especially thosewhich exhibit a tensile strength greater than about 300×10³ psi, atensile modulus greater than about 35×10⁶ psi, and are stable in aninert atmosphere at temperatures up to about 1400° C. While monofilamentfibers may be used, multifilament carbon yarns are preferred. Amultifilament carbon yarn with an average filament diameter of about 6microns (μm) to about 10 μm, and particularly about 7 μm to about 10 μmis especially preferred. Suitable carbon yarns include HMU™,manufactured by Hercules Corporation (Wilmington, Del.); P-100,manufactured by Amoco Performance Products (Ridgefield, Conn.); andFORCA FT700, manufactured by Tonen Corporation (Tokyo, Japan). The HMU™yarn is available with 1000, 3000, 6000, or 12,000 fibers per tow and anaverage fiber diameter of 8 microns. It has a tensile strength of400×10³ psi, a tensile modulus of 55×10⁶ psi, a CTE of -7×10⁻ 7 cm/cm°C., and a density of 1.84 g/cm³. The P-100 fiber has a tensile strengthof 325×10³ psi, a tensile modulus of 105×10⁶ psi, a CTE of -16×10⁻⁷cm/cm °C., and a density of 2.16 g/cm³. The FT700 yarn has a tensilestrength of 500×10³ psi, a tensile modulus of 100×10⁶ psi, a CTE of-1.5×10⁻⁶ cm/cm °C., and a density of 2.14 g/cm³. The HMU™ fiber ispreferred for discontinuously reinforced composites because of itsinherently higher failure strain.

Boron nitride (BN) reinforcing particles are suitable for this inventionbecause they resist oxidation and have a CTE less than that of thematrix material. The fact that they do not form significant chemicalbonds with the matrix material is of particular importance. Significantchemical bonds means bonds resulting from reaction with the matrix orpartial dissolution in the matrix. Particles which form strong bondswith the matrix material are subject to the full stress applied to thematrix and tend to fracture rather than to blunt cracks. The BNreinforcing particles only form weak physical bonds with the matrixmaterial, a property which can be characterized by the fact that BNparticles are not significantly wetted by the matrix material. Particleswhich are significantly wetted by the matrix material tend to formstronger physical or chemical bonds with the matrix material and wouldprobably not be suitable for this invention. Particles of materialsother than BN which resist oxidation, have a CTE less than the matrixmaterial, and do not form significant chemical bonds with the matrixmaterial would also be suitable for the practice of the presentinvention.

BN particles of various shapes, including rods, discs, platelets, orspheres are suitable for use with the present invention. Preferably, theparticles will have an aspect ratio of at least 5:1 and will be no morethan about one half the diameter of the carbon fibers in any direction.For disc-shaped particles, for example platelets, an aspect ratio ofabout 5:1 provides a normalized toughening increment of 1.25-1.5. Thenormalized toughening increment is a theoretical parameter in which 1.0indicates no matrix toughening, while values greater than 1.0 indicatethe relative degree of matrix toughening. Higher aspect ratios wouldprovide even greater normalized toughening increments. Most preferably,the particles will be no more than about 10% to about 40% of thediameter of the carbon fibers in any direction. For best results, theparticles should be homogeneously distributed throughout the matrixmaterial. Preferably, the particles will be distributed randomly. BNplatelets having a diameter of about 0.5 μm to about 2.0 μm diameter anda thickness of about 0.10 μm or less have been found to be particularlysuitable for use in carbon fiber reinforced borosilicate glasscomposites in which the carbon reinforcing fibers have a diameter ofabout 7 μm to about 10 μm. Such platelets are available from Cerac(Milwaukee, Wis.) as hexagonal boron nitride, from Union CarbideAdvanced Ceramics (Cleveland, Ohio) as HCP boron nitride, HPF boronnitride or MW-5 boron nitride, from Standard Oil Engineered Materials(Niagra Falls, N.Y.) as Combat® boron nitride, and from ESK EngineeredCeramics (New Canaan, Conn.) as Type S boron nitride.

A CFRGM composite article containing BN reinforcing particles may beformed in any of the ways known to the art for forming similarcomposites which do not contain reinforcing particles. Hot pressing andinjection molding are the preferred methods. The key difference betweenthe methods taught in the art and the methods required to practice thecurrent invention is the incorporation of reinforcing particles into theCFRGM composite.

The BN reinforcing particles may be incorporated into the CFRGMcomposite in several ways. For example, the BN particles may bemechanically mixed with prepregged, chopped carbon fibers. The preferredmethod is to incorporate the reinforcing particles directly into thecarbon fiber tow along with the glass powder during the prepreggingprocess. This method results in uniform distribution of the BNreinforcing particles between the prepregged carbon fibers andeliminates any mixing which may result in segregation of theconstituents.

First, a slurry of glass powder, BN reinforcing particles, and a carrierliquid should be prepared. The BN reinforcing particles should be welldispersed in the slurry so that they will be homogeneously distributedthroughout the matrix of the composite. Preferably, the glass powderwill be about -325 mesh and the reinforcing particles will be about 1 μmin diameter. The preferred carrier is water, although any liquidcompatible with the binder to be added in the next step may be used.Appropriate amounts of glass powder and BN particles are added to anappropriate amount of carrier. While the amount of materials in theslurry may vary, the amount of glass added should be adequate to giveabout 15 volume percent (vol %) to about 40 vol % fibers and about 10vol % to about 25 vol % reinforcing particles when the carrier liquidand binder are removed. The final molded article generally containsabout 40 vol % to about 75 vol % glass matrix. The preferred amount ofglass and BN particles will depend on the particular application. Theresulting mixture is shaken vigorously for a few minutes to initiallydisperse the glass powder and BN particles in the carrier. Shakingalone, however, is insufficient to adequately disperse the BN particlesbecause they tend to agglomerate due to their shape and extremely smallsize. Therefore, after initially dispersing the glass powder and BNparticles by shaking, the glass powder/BN particle/carrier mixtureshould be milled. Preferably, the mixture will be milled with anultrasonic mixer for about 15 minutes or until it is evident that any BNagglomerates are broken up and uniformly dispersed in the slurry. AModel VT600 Vibra-Cell high intensity ultrasonic processor, availablefrom Sonics & Materials (Danbury, Conn.), is a suitable ultrasonicmixer. Operating the mill at 70% power and a 50% duty cycle (on-offcycle of 1:1) has been found to be particularly effective.

After the glass powder/BN particle/carrier slurry has been milled, abinder should be added to hold the glass powder and BN particles inplace within the fiber tow during subsequent cutting or choppingoperations. The binder should not be added before milling because thiswould cause excessive foaming of the mixture. The binder may be any ofthe binders customarily used to prepreg carbon fibers. For example, thebinder may be a polymeric binder which dissolves or disperses readily inthe carrier. Preferred polymeric binders include latex-acrylic typepolymers, such as Rhoplex™ latex-acrylic which is available from Rohm &Haas Corporation (Philadelphia, Pa.), and the Carbowax™ series ofpolymers, such as Carbowax 4000 which is available from Union CarbideCorporation (Danbury, Conn.). Alternately, the binder may be aninorqanic binder which dissolves or disperses readily in the carrier.Preferred inorganic binders include colloidal silica solutions, such asLudox™ which is available from E.I. DuPont de Nemours (Wilmington,Del.).

Once the glass powder/BN particle/carrier/ binder slurry has beenprepared, carbon fiber tows are drawn through the slurry in such a waythat the slurry saturates the fiber tow. When the proper proportions ofthe slurry constituents are employed, the fiber rows will be impregnatedwith an amount of glass sufficient to bring the volume fraction of glassinto the desired 40 vol % to 75 vol % range. The impregnated tows arethen wound onto a mandrel and dried.

The dried tows are chopped to a length useful with the intended moldingprocess. For example, if the fibers are to be used in an injectionmolding process, they should be cut short enough to prevent clumpingwhen passing through an orifice into a mold. Preferably, fibers used forinjection molding will be about 0.635 cm to about 1.27 cm in length.Finally, the impregnated tows are formed into desired articles by any ofthe methods known to the art for forming CFRGM composites.

EXAMPLE

298 meters of HMU™ Magnamite™ graphite fibers (Hercules Incorporated,Wilmington, Del.), having 3000 filaments per tow and a weight of 65grams were impregnated with a glass powder/BN particle/water/binderslurry by unrolling the fiber yarn from a feed spool, removing the fibersizing by passing the yarn through the flame of a bunsen burner atmoderate speed, about 6.5 meters/min, immersing the yarn in an agitatedslurry, and winding the saturated yarn on a take-up mandrel. The slurryconsisted of 300 grams of -325 mesh Corning Code 7070 borosilicate glasspowder (Corning Glass Works, Corning, N.Y.), 100 grams of hexagonalboron nitride platelets (Cerac, Milwaukee, Wis.), 50 grams of Ludox™colloidal silica binder (E.I. DuPont de Nemours, Wilmington, Del.),and600 ml of distilled water. Sufficient slurry impregnated the yarn to add259 grams of glass powder and 87 grams of boron nitride. The saturatedyarn was dried on the take-up mandrel to remove the water.

After drying, the impregnated yarn was removed from the take-up mandreland cut to an average tow length of 1.25 cm. The chopped prepreggedfibers, known as molding compound, were placed in the reservoir chamberof an injection molding apparatus which had an injection port with awidth of 0.51 cm and a length of 7.5 cm. The injection molding apparatuswas placed in a vacuum hot press and heated to 1275° C. A loadcalculated to apply 7 MPa to the plunger was applied and maintained for30 minutes. Furnace power was shut off and the assembly cooled to 500°C. at which point the pressure was removed. The assembly was cooled toroom temperature and the injection molded part was removed from themold.

The Figure, a scanning electron micrograph, shows well distributed BNplatelets in a discontinuous CFRGM composite made according to theExample. The platelets appear as small white particles dispersed amongthe much larger carbon fibers. The BN platelets are homogeneouslydistributed throughout the matrix, including between individual carbonfibers, and are random in their spatial orientation, as opposed to beingaligned or preferentially oriented. The majority of the platelets existas discrete particles, rather than in large agglomerations of severalplatelets. A BN particle distribution similar to that shown in theFigure is preferred because it provides a more uniform material withvery few matrix-rich regions which are not reinforced by either carbonfibers or reinforcing particles. As a result, the composite has atougher matrix which demonstrates improved oxidative stability. Whilethe BN particle distribution shown in the Figure is preferred, otherparticle distributions, such as a less random, more agglomerateddistribution would also provide some of the benefits of this invention.

                  TABLE                                                           ______________________________________                                                        Conventional                                                                           Composite                                                            Composite                                                                              + BN                                                 ______________________________________                                        Apparent Porosity  0.60       0.18                                            As-pressed (%)                                                                Normalized Ave. Weight Loss                                                                     32.44       8.76                                            After Oxidation (mg/cm.sup.2)                                                 Average Flexural Strength                                                     As-pressed (ksi)  19.1       26.5                                             After Oxidation (ksi)                                                                           5.5        23.0                                             Coefficient of Friction                                                                          0.273      0.174                                           As-pressed                                                                    ______________________________________                                    

The Table compares properties of a sample of a conventional CFRGMcomposite which does not have BN reinforcing particles (ConventionalComposite) with those of a similar CFRGM composite with about 17 vol %BN platelets made according to the Example (Composite+BN). Bothcomposites had a Corning Code 7070 glass matrix and about 25 vol %carbon fibers. The apparent porosity, which is a measure of the sample'ssurface-connected porosity resulting from matrix microcracking, is agood indicator of the relative volume available to transport oxygen tothe interior of the composite because surface cracks tend to link upwith interior microcracks. As shown in the Table, the CFRGM compositewhich contained BN platelets had an apparent porosity of less thanone-third that of the conventional CFRGM composite. As a result, thecomposite containing BN platelets would be expected to display muchbetter oxidative stability than the conventional composite.

The figures reported for normalized average weight loss and flexuralstrength demonstrate the improved oxidative stability of the CFRGMcomposite containing BN platelets over the conventional CFRGM composite.Both composites were exposed to flowing oxygen at 800° F. andatmospheric pressure for 500 hours. Weight loss, which is due primarilyto the oxidation of carbon fibers, is indicative of the extent of fiberloss. The lower weight loss in the composite containing BN is due to theless extensive microcracking in the composite's matrix. As to beexpected from a lower oxidation rate, the composite containing BNretained much more of its flexural strength than did the composite whichdid not contain BN. Also of interest is the fact that the compositecontaining BN had a higher as-pressed, that is, not oxidized, flexuralstrength than the conventional composite. The superior strength resultsfrom the presence of BN in the matrix, which provides additionalreinforcement to the composite.

The coefficient of friction data show that a CFRGM composite containingBN platelets displays improved lubricity over a conventional CFRGMcomposite. These data were obtained at room temperature on a RheometricsModel RDS II rheometer by contacting as-pressed composites against anInconel 718 counterface at various rotational rates and loadings. Datawere taken under both dynamic (oscillatory) and steady-state (gliding)conditions. The reported coefficients of friction are an average of thedata at each rotational rate. The conventional composite's lowcoefficient of friction is due to carbon reinforcing fibers, which areknown to have good lubricating properties, exposed on the composite'ssurface. Like the carbon reinforcing fibers, BN has a graphitic crystalstructure which also gives it good lubricating properties. As a result,the addition of BN to the CFRGM composite results in a composite withimproved as-pressed lubricity. The BN-containing CFRGM composite shoulddisplay an even more significant improvement in lubricity over theconventional CFRGM composite after the composites are oxidized.Oxidation will destroy a significant number of the surface-exposedcarbon fibers in the conventional composite, resulting in a substantialloss in lubricity. On the other hand, the BN containing composite willretain more of its carbon fibers, and therefore, lose less of thelubricity attributable to the fibers. Moreover, because BN oxidizes atleast 100 times slower than carbon fibers, almost all of the lubricityattributable to the BN should be retained.

As can be seen from the data in the Table, the addition of BN particlesto a CFRGM composite improves the composite's properties in at least twoways. First, the BN particles themselves act as a secondary reinforcingand lubricating phase. Second, the BN particles reduce the effects ofcarbon fiber oxidation by suppressing the formation of microcracks inthe matrix. One way in which the BN particles suppress microcracks is byblunting the microcracks as they form, thereby toughening the matrix andpreventing the development of extensive channels by which oxygenpenetrates to the carbon reinforcing fibers. Another way in which BNparticles suppress microcracking is by reducing the thermal mismatchbetween the "matrix" and the carbon fibers, where the "matrix" isconsidered to be the combination of the glass and the BN particles. Ineffect, the BN particles, which have a lower CTE than the glass,decrease the overall CTE of the "matrix". Because there is less of athermal expansion mismatch between the "matrix" and the carbon fibers,fewer microcracks form due to thermal stresses.

An article fabricated from a discontinuously reinforced CFRGM compositecontaining BN particles will display improved oxidative stability,lubricity, matrix toughness, strength, stiffness, and machinability oversimilar CFRGM composites which do not contain BN particles. A personskilled in the art would be able to identify many uses of a composite ofthe present invention.

It should be understood that the invention is not limited to theparticular embodiment shown and described herein, but that variouschanges and modifications may be made without departing from the spiritor scope of the claimed invention.

I claim:
 1. A method of fabricating a discontinuous carbon fiberreinforced glass matrix composite, comprising:(a) mixing a glass powderand boron nitride reinforcing particles in a carrier liquid to create aslurry; (b) adding a binder to the slurry; (c) impregnating a continuousmultifilament carbon fiber yarn with the slurry; (d) drying theimpregnated carbon fiber yarn to remove the carrier liquid; (e) cuttingthe impregnated carbon fiber yarn to a suitable length; and (f) moldingthe impregnated carbon fiber yarn in a suitable molding means to form adiscontinuous carbon fiber reinforced glass matrix composite.
 2. Themethod of claim 1 wherein the molding means is a hot pressing apparatus.3. The method of claim 1 wherein the molding means is an injectionmolding apparatus.
 4. The method of claim 1 wherein the glass powder andboron nitride particles are mixed with an ultrasonic mixing device. 5.The method of claim 1 wherein the glass powder comprises a borosilicateglass.
 6. The method of claim 1 wherein the boron nitride reinforcingparticles have an aspect ratio of at least 5:1.
 7. The method of claim 1wherein sufficient quantities of boron nitride particles, carbon fiberreinforcing fibers, and glass powder are used to produce a compositewith about 10 volume percent to about 25 volume percent boron nitrideparticles.